In weapons related target pointing and tracking (P&T) systems, laser beam energy must be transmitted through the earth's atmosphere or other aberrating media which may distort the beam radiation. Further, in such systems, the P&T system must compensate for relative motion of the tracking system and the target. With respect to distortion compensation, two techniques have been employed known as "conventional" and "unconventional". "Conventional" adaptive optics schemes use wavefront sensors combined with deformable mirrors. "Unconventional" adaptive optics schemes are primarily based on all-optical phase conjugation using nonlinear optics devices. Both techniques are reviewed in "Principles Of Adaptive Optics", by R. J. Ryson, Ch.3 (Academic Press 1991).
A third method for distortion compensation is referred to as "Holographic Interactive Tracker" (HIT). The operation of a HIT system comprises a first acquisition step and a second engagement step. In order to implement the HIT system, the target is typically detected and its approximate position known through the use of separate acquisition systems. The HIT system utilizes the information obtained from these other systems to complete the acquisition step.
In the acquisition step, a wide acquisition beam is directed to a known target. The frequency of this beam is locked to a master oscillator. A fraction of the diverging energy from the acquisition beam reflected by the target is returned as a beam or as diffuse radiation. The returned radiation is collected by a receiver and is interfered with a local oscillator beam on a charge-coupled-device detector array (CCD), thereby forming an electro-optic hologram. The hologram is processed, and the resulting pattern is transferred to a spatial light modulator (SLM) where the electronic hologram is written in as a phase-hologram.
In the engagement step of the HIT System, a beam from the same or a separate laser oscillator is reflected off the SLM. The SLM then outputs multiple beams of radiation. Most of the energy is contained within an image beam a conjugate image beam. Through the properties of phase-conjugation, the conjugate beam retraces the target return path while at the same time the wave-front distortions are undone, resulting in maximum energy delivery to the target. By repeating the acquisition and engagement steps as the target moves, the HIT system automatically tracks it.
The HIT system therefore provides automatic target acquisition and tracking as well as distortion compensation. The HIT System can also form the basis of a secure one-way or two-way communication system between stationary or moving platforms. The secure communication link can also be used for vehicle guidance, transmitting guidance commands along the narrow beam to the moving vehicle.
Reference may be had to the following patent for further information concerning the state of the technology relating to a HIT System: U.S. Pat. No. 5,378,888, issued Jan. 3, 1995 entitled "Holographic System For Interactive Target Acquisition And Tracking" issued to Stappaerts.
It is desirable in certain laser applications, such as directed energy weapons (DEW) and vibration sensing lidars, that a small spot be formed on a selected part of a target, and that it be maintained there during an engagement time, even in the presence of relative motion and/or optical distortions due to, for example, atmospheric turbulence. In DEW applications, a smaller spot size results in increased energy density on target and, therefore, increased effectiveness. In vibration sensing, a small spot size at the optimum target location is desired to maximize the vibration signal strength. It would therefore be desirable for a HIT System to be able to meet the requirements of such systems.